Active-active architecture for distributed ISCSI target in hyper-converged storage

ABSTRACT

A method is provided for a hyper-converged storage-compute system to implement an active-active failover architecture for providing Internet Small Computer System Interface (iSCSI) target service. The method intelligently selects multiple hosts to become storage nodes that process iSCSI input/output (I/O) for a target. The method further enables iSCSI persistent reservation (PR) to handle iSCSI I/Os from multiple initiators.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a 371 application of InternationalApplication PCT/CN2018/115837, filed Nov. 16, 2018 and entitled“ACTIVE-ACTIVE ARCHITECTURE FOR DISTRIBUTED ISCSI TARGET INHYPER-CONVERGED STORAGE.” The International application. The presentapplication is also related to U.S. patent application Ser. No.15/350,027, filed Nov. 12, 2016, which is commonly owned andincorporated by reference.

BACKGROUND

VMware vSphere is suite of virtualization software for implementing andmanaging virtual infrastructures. The software includes (1) ESXihypervisor that implements virtual machines (VMs) on physical hosts, (2)virtual storage area network (vSAN) that aggregates local storage toform a shared datastore for a cluster of ESXi hosts, and (3) vCenterServer that centrally provisions and manages virtual datacenters, VMs,ESXi hosts, clusters, datastores, and virtual networks. The vSANsoftware may be implemented as part of the ESXi hypervisor software.

The vSAN software uses the concept of a disk group as a container forsolid-state drives (SSDs) and non-SSDs, such as hard disk drives (HDDs).On each host (node) in a vSAN cluster, the local drives are organizedinto one or more disk groups. Each disk group includes one SSD thatserves as read cache and write buffer (e.g., a cache tier), and one ormore SSDs or non-SSDs that serve as permanent storage (e.g., a capacitytier). The aggregate of the disk groups from all the nodes form a vSANdatastore distributed and shared across the nodes.

The vSAN software stores and manages data in the form of data containerscalled objects. An object is a logical volume that has its data andmetadata distributed across a vSAN cluster. For example, every virtualmachine disk (VMDK) is an object, as is every snapshot. For namespaceobjects, the vSAN software leverages virtual machine file system (VMFS)as the file system to store files within the namespace objects. Avirtual machine (VM) is provisioned on a vSAN datastore as a VM homenamespace object, which stores metadata files of the VM includingdescriptor files for the VM's VMDKs.

vSAN introduces a converged storage-compute platform where VMs arerunning on ESXi hosts as usual while a small percentage of CPU andmemory resources is used to serve the storage needs of the same VMs.vSAN enables administrators to specify storage attributes, such ascapacity, performance, and availability, in the form of simple policieson a per-VM basis. vSAN offers many advantages over traditional storage,including scalability, simplicity, and lower total cost of ownership.

Internet small computer system interface (iSCSI) is a transport layerprotocol that describes how small computer system interface (SCSI)packets are transported over a transmission control protocol/Internetprotocol (TCP/IP) network. By using standard Ethernet, iSCSI is cheaperto adopt and easier to manage because it does not require expensive andsometimes complex switches and cards that are needed to run FibreChannel (FC) networks. In recent years, the transport speed of iSCSI hasbecome comparable to FC. As a result, iSCSI has gained popularity suchthat most platforms that use block devices support the iSCSI protocol.

vSAN iSCSI target (VIT) service allows hosts and physical workloads thatreside outside a vSAN cluster to access a vSAN datastore. VIT serviceenables an iSCSI initiator on a remote host to transport block-leveldata to an iSCSI target on a storage device in the vSAN cluster. Afterenabling and configuring VIT service on the vSAN cluster, a user candiscover iSCSI targets from the remote host using the Internet protocol(IP) address of any ESXi host in the vSAN cluster and the TCP port ofthe iSCSI targets. To ensure high availability of the iSCSI targets, theuser may use the IP addresses of two or more ESXi hosts to configuremultipath support for his or her iSCSI application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a hyper-converged, distributedstorage system having an active-passive (AP) failover architecture forVIT service.

FIG. 2 is a block diagram illustrating a hyper-converged, distributedstorage system in some examples of the present disclosure.

FIG. 3 is a block diagram illustrating distributed storage modules ofhypervisors in the system of FIG. 2 in some examples of the presentdisclosure.

FIG. 4 is a block diagram illustrating the system of FIG. 2 having anactive-active (AA) failover architecture for VIT service in someexamples of the present disclosure.

FIG. 5 is a block diagram illustrating the placement of discovery nodesand storage nodes on hosts in relation to the component locations oftarget namespace objects in the system of FIG. 2 in some examples of thepresent disclosure.

FIG. 6 is a block diagram illustrating additional details of the systemin FIG. 2 in some examples of the present disclosure.

FIG. 7 is a block diagram illustrating additional details of a host inthe system of FIG. 2 in some examples of the present disclosure

FIG. 8 is a block diagram illustrating a namespace to manage targets andLUNs in the system of FIG. 2 in some examples of the present disclosure.

FIG. 9 is a block diagram illustrating a flowchart of a method for thesystem of FIG. 2 to implement AA failover architecture for VIT servicein some examples of the present disclosure.

FIG. 10 is a block diagram illustrating an iSCSI communication sequencein the system of FIG. 2 in some examples of the present disclosure.

FIG. 11 is a block diagram illustrating a method for the system of FIG.2 to process iSCSI persistent reservation (PR) requests in some examplesof the present disclosure.

FIG. 12 is a block diagram illustrating a method for the system of FIG.2 to process PR requests in some examples of the present disclosure.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof. In the drawings,similar symbols typically identify similar components, unless contextdictates otherwise. The illustrative embodiments described in thedetailed description, drawings, and claims are not meant to be limiting.Other embodiments may be utilized, and other changes may be made,without departing from the spirit or scope of the subject matterpresented here. It will be readily understood that the aspects of thepresent disclosure, as generally described herein, and illustrated inthe Figures, can be arranged, substituted, combined, and designed in awide variety of different configurations, all of which are explicitlycontemplated herein.

Virtual storage area network (vSAN) iSCSI target service encompassesfour basic concepts: (1) target, (2) logical unit number (LUN), (3)discovery node (DN), and (4) storage node (SN).

A target is a container for LUNs. It is the unit for authentication andauthorization as well as failover. An initiator connects to a target andthen accesses the LUNs in the target. A target is implemented as anamespace object created by the vSAN software from storage in the vSANdatastore.

A LUN is a block device that can be consumed by the initiator. A LUN isimplemented as a virtual machine disk (VMDK) created by the vSANsoftware from the vSAN datastore as described in co-pending U.S. patentapplication Ser. No. 15/350,027 filed Nov. 12, 2016, which is commonlyowned and incorporated by reference.

A DN is an ESXi host that can act as a discovery portal for iSCSIservice that an initiator may access to discover available targets.

A SN is an ESXi host that can process iSCSI input/outputs (I/Os) to theLUNs within a target.

FIG. 1 is a block diagram illustrating a hyper-converged, distributedstorage system 10 having an active-passive (AP) failover architecturefor vSAN iSCSI target (VIT) service. System 10 includes a cluster ofESXi hosts 12-1, 12-2, 12-3, and 12-4 (collectively as “ESXi hosts 12”or individually as a generic “ESXi host 12”). ESXi hosts 12-1, 12-2,12-3, and 12-4 run respective ESXi hypervisors 13-1, 13-2, 13-3, and13-4, which have vSAN modules (not shown) that implement a vSAN layer 16to create a vSAN datastore (not shown). ESXi hypervisors 13-1, 13-2,13-3, and 13-4 also have respective VIT modules 18-1, 18-2, 18-3, and18-4 that implement a VIT layer 19 to create iSCSI targets (not shown)from the vSAN datastore.

In the active-passive (AP) failover architecture for the current VITservice, each target is owned by a single ESXi host 12 (morespecifically its hypervisor 13 or VIT software component), and everyESXi host 12 acts as a discovery portal for iSCSI service. The targetowner is responsible for opening all the VMDKs in its target andproviding access to LUNs backed by the VMDKs. If an iSCSI initiator 14attempts to connect to a non-owner host for a target, the non-ownerwould redirect initiator 14 to the target owner via iSCSI redirect. Forexample, when initiator 14 attempts to connect to ESXi hosts 12-1 and12-4 for the target owned by ESXi host 12-2, ESXi hosts 12-1 and 12-4redirect initiator 14 to ESXi host 12-2. Even if initiator 14 attemptsto use multipathing to create multiple connections to both the targetowner and one or more of the non-owners, the non-owners would redirectinitiator 14 to the target owner via iSCSI redirect.

When a target owner fails, vSAN layer 16 selects a new target owner. Atarget owner fails when the host loses power, crashes, reboots, or losesnetwork connectivity. At any given time, there is only a single activeESXi host 12 for a target, and the other ESXi hosts 12 serve as backup.In summary, every ESXi host 12 is a DN for every target, and there isonly a single SN for a target. System 10 is described in greater detailin co-pending U.S. patent application Ser. No. 15/350,027 filed Nov. 12,2016, which is commonly owned and incorporated by reference.

One drawback of the AP failover architecture for the current VIT serviceis its failover time. When failover occurs, a new target owner takesseveral seconds to open all the VMDKs backing the LUNs in a target andinitialize the states for all the LUNs. The failover time is broken downto four main parts.

First is the delay for the vSAN layer to detect an ESXi host's“liveness” that indicates if the host is connected to the other ESXihosts in a vSAN cluster. The vSAN layer detects host liveness every 7seconds so this delay could be anywhere from 0 to 7 seconds.

Second is the 16-second virtual machine file system (VMFS) lock timeout.VMFS provides a distributed lock mechanism that restricts access to afile shared by EXSi hosts. An “exclusive” lock prevents multiple ESXihosts from concurrently writing a file and corrupting metadata. If anESXi host holding an exclusive lock on a file does not renew the lockwithin the 16-second timeout, another ESXi host may break the lock andplace its own lock on the file. The heartbeat region of a VMFS volume isused for this on-disk locking, and every host that uses the VMFS volumehas its own heartbeat region. This region is updated by the host onevery heartbeat (e.g., 3 seconds). The region that is updated is thetime stamp, which tells others that this host is alive. When the host isdown, this region is used to communicate lock state to other hosts.

VIT service leverages the VMFS lock mechanism to coordinate opening andwriting LUNs. To open and write a LUN, a target owner acquires theexclusive lock on a descriptor file of the LUN in a target namespaceobject, wherein the descriptor file has the vSAN universal uniqueidentifier (UUID) of a VMDK object backing the LUN. When the targetowner dies suddenly, a new target owner must wait at least 16 secondsfor the prior target owner's heartbeat, which is established per target(VMFS volume), to time out before the new target owner may open the VMDKobject backing the LUN. When several targets fail together, the recoveryprocess must wait for the VMFS heartbeat of each target to time out.While the timeouts may occur in parallel, the overall wait time stillincreases when the number of targets rises.

Third is the time for a new target owner to open a LUN (VMDK) afterestablishing its VMFS heartbeat. Fourth is the time for a new targetowner to initialize a LUN (VMDK) for input/output (I/O) processing.While both these times are short, the overall wait still increases whenthe number of LUNs (VMDKs) increases.

Another drawback of the AP failover architecture for the current VITservice is its performance. The iSCSI protocol stack on a target ownerbecomes a bottleneck when there are many LUNs (e.g., 100 LUNs) in atarget because a single target owner processes all I/Os to the LUNs inthat target. The network bandwidth of the target owner also forms abottleneck.

FIG. 2 is a block diagram illustrating a simplified view ofhyper-converged, distributed storage system 200 in some examples of thepresent disclosure. System 200 includes host computers 202-1, 202-2,202-3, . . . 202-i (collectively as “hosts 202” or individually as ageneric “host 202”) running respective (e.g., ESXi) hypervisors 204-1,204-2, 204-3, and 204-4 (collectively as “hypervisors 204” orindividually as a generic “hypervisor 204”) to provide a hyper-convergedcompute layer 206 and a hyper-converged, distributed storage (e.g.,vSAN) layer 208. Hypervisors 204-1, 204-2, 204-3, and 204-4 includerespective hyper-converged, distributed storage (e.g., vSAN) modules210-1, 210-2, 210-3, and 210-4 (collectively as “vSAN modules 210” orindividually as a generic “vSAN module 210”) that implement vSAN layer208. A management (e.g., vCenter) server 212 centrally provisions andmanages (1) VMs 214 on the hyper-converged compute layer 206 and (2) ahyper-converged distributed (e.g., vSAN) datastore 222 on vSAN layer208. Hosts 202 make up nodes of a hyper-converged, distributed storage(e.g., vSAN) cluster 216. Nodes 202 contribute local storage resources(e.g., non-SSDs 218 and SSD 220) to form vSAN datastore 222.

FIG. 3 is a block diagram illustrating vSAN modules 210 in some examplesof the present disclosure. Each vSAN module 210 includes the followingcomponents: cluster monitoring membership and directory services (CMMDS)302, cluster level object manager (CLOM) 304, distributed object manager(DOM) 306, local log structured object management (LSOM) 308, andreliable datagram transport (RDT) 310. CLOM 304 validates resourceavailability and ensure objects are placed in vSAN cluster 216 in a waythat satisfy their storage policies, and DOM 306 creates components andapplies configuration locally through LSOM 308. DOM 306 also coordinateswith its counterparts for component creation on other nodes 202 in vSANcluster 216. All subsequent reads and writes to objects funnel throughDOM 306, which will forward them to the appropriate components. RDT 310is the communication mechanism for VMDK input/output (I/O) in a network312 between nodes 202. It uses transmission control protocol (TCP) atthe transport layer and it is responsible for creating and destroyingTCP connections (sockets) on demand.

Each object has a unique owner DOM 306 at any given time. The owner DOM306 is responsible for monitoring the storage policy compliance for theobject. In case the owner DOM 306 detects a component for the objectfails, it would wait for a specified time (e.g., 60 minutes), and thenit asks CLOM 304 to re-configure the object according to its storagepolicy. CLOM 304 would tell owner DOM 306 where to place the newcomponent if re-configuration is needed. Owner DOM 306 would create anew component and resync the data to the new component. If owner DOM 306dies, the other DOMs 306 would leverage CMMDS to elect a new owner DOMfor the objects of the failed DOM.

CMMDS 302 is responsible for monitoring the vSAN cluster's membership,checking heartbeats between nodes 202, and publishing updates to thecluster directory. Other software components use the cluster directoryto learn of changes in cluster topology and object configuration. Forexample, DOM 306 uses the contents of the cluster directory to determinenodes 202 storing the components of an object and the paths by whichthose hosts 202 are reachable.

To handle failures, each host 202 plays one of three roles in the VSANcluster: master, agent or backup. Roles are applied during a clusterdiscovery when all hosts 202 participating in the VSAN cluster elect amaster. The master node is responsible for getting clustering serviceupdates from other nodes and distributing these updates to the othernodes through its connections with the other nodes. The clusteringservice updates include changes to object and component ownership, suchas when a prior owner of an object dies and a new owner is selected. Thebackup node assumes the master role when the master node fails. Theagent nodes may change roles when both the master and the backup nodefail. Each agent node accepts local cluster service updates, such aschanges to object and component ownership, and forwards them to themaster node for distribution to the other nodes. It also accepts updatesfrom the master node and applies them to the local copy of the clusterdirectory.

FIG. 4 is a block diagram illustrating system 200 having anactive-active (AA) architecture for VIT service in some examples of thepresent disclosure. Hypervisors 204-1, 204-2, 204-3, and 204-4 includerespective vSAN modules 210-1, 210-2, 210-3, and 210-4 that implementvSAN layer 208. Hypervisors 204-1, 204-2, 204-3, and 204-4 also includerespective iSCSI target (e.g., VIT) modules 402-1, 402-2, 402-3, and402-4 (collectively as “VIT modules 402” or individually as a generic“VIT module 402”) that implement an iSCSI target (e.g., VIT) layer 404.

In the AA architecture, instead of a single target owner serving iSCSII/Os for a target, multiple hosts simultaneously serve iSCSI I/Os for atarget. For example, hosts 202-2 and 202-3 concurrently serve iSCSI I/Osfor a target (not shown). In other words, a target has multiple SNs.Each host 202 acts as a DN for every target. If an iSCSI initiator 406attempts to connect to a target through a non-SN of the target, thenon-SN would randomly or sequentially (e.g., one after another) selectone of the SNs of the target and redirect initiator 406 to the selectedSN via iSCSI redirect. For example, when initiator 406 attempts toconnect to hosts 202-1 and 202-4 (via multipathing) for the targetserved by hosts 202-2 and 202-3, hosts 202-1 and 202-4 randomly selectand redirect initiator 406 to hosts 202-2 and 202-3, respectively. Toevenly distribute the initiator's connections among the SNs for thetarget, any SN that receives a redirect may determine heuristically tofurther redirect the initiator based on the number of connections it hasfor an <initiator, target> pair or its own workload. For example, eachSN can be configured to only support one or a few connections for an<initiator, target> pair so that the SN would randomly select andredirect the initiator to another SN when the threshold is exceeded.

To take advantage of multiple SNs for a target, an initiator utilizesmultipathing to send I/Os through the SNs to the target. System 200 doesnot need to be concerned about the order of the I/Os arriving at atarget because multipathing does not guarantee the order of the I/Osfrom different paths. To guarantee data consistency in system 200, vSANlayer 208 (more specifically CMMDS 302 and DOM 306 in hosts 202) ensuresthat writes to one host 202 are immediately visible to the other hosts202 so each host 202 knows if it may use the data in its or if it mustretrieve the data from another host 202 (e.g., DOM 306 does not uselocal cache when multi-writer is enabled).

A cluster of initiators may access the same LUN in a target. To ensuredata consistency, initiators in a cluster may use SCSI persistentreservations (PR). Thus, as described later, system 200 supports SCSIPR.

VMFS provides a multi-writer mechanism that allows multiple VMs to openand write a VMFS-backed virtual disk (e.g., VMDK) by setting amulti-writer flag for the VMDK. The same mechanism allows multiple hoststo open and write to the same LUN backed by a VMDK. In theory, each host202 may open all the LUNs in a distributed (e.g., vSAN) datastore andact as a SN for all the LUNs in every target so no iSCSI redirect isneeded. However, this approach suffers from scalability issues. If ahost 202 opens all the LUNs in every target, it would consume memory forall the LUNs even if no initiator accesses the LUNs through this host202. If the number of LUNs increases, every host 202 would consume moreresources. The total number of targets supported by a cluster would belimited to the total number of targets supported by the hostsindividually. Furthermore, this approach offers no method to balanceiSCSI initiators to different hosts 202. Thus, there is a need for amethod to intelligently select SNs for a target.

In examples of the present disclosure, the number of SNs for a target isselected to satisfy the number of host and device failures to tolerate(FTT) for the target and its LUN. In the context of a distributedstorage system such as vSAN, failures to tolerate is defined as thenumber of vSAN nodes that could fail while still being able to provideaccess to all the data that is stored in vSAN. Thus, if the target has Nnumber of FTT, then the number of SNs for the target would be at leastN+1 to guarantee fault tolerance for VIT service.

VIT layer 404 creates a namespace object for every target, and vSANlayer 208 creates components for the target namespace object and placesthem among hosts 202 in the cluster according to the target's FTT andfailure tolerance method. In some examples of the present disclosure,any host 202 that owns a component of a target namespace object becomesa SN for that target.

As component placement is used for selecting SNs for each target, RAID-1is used as the failure tolerance method for all the targets so theircomponent placement has a fixed pattern. Furthermore, a target's FTT isadjusted automatically according to the maximum FTT of the LUNs in thetarget. For example, if a target's FTT is set to one (1), then it wouldhave two duplicate components and two hosts would become SNs for thetarget. When a target's FTT increases, the number of components as wellas the number of SNs increase. For example, if a target's FTT isincreased to two (2), then it would have three duplicate components andthree hosts would become SNs for the target.

FIG. 5 is a block diagram illustrating the placement of DNs and SNs onhosts 202 in relation to the component locations of target namespaceobjects in system 200 in some examples of the present disclosure. System200 is shown with three targets 502-1, 502-2, and 502-3 (collectively as“targets 502” or individually as a generic “target 502”). When targets502 have a FTT of one (1) and use RAID-1 as the failure tolerancemethod, each target 502 would have two (2) duplicate components storedon two different hosts 202. For example, target 502-1 has duplicatecomponents 504-11 and 504-12 stored on hosts 202-1 and 202-2,respectively, target 502-2 has duplicate components 504-21 and 504-22stored on hosts 202-2 and 202-3, respectively, and target 502-3 hasduplicate components 504-31 and 503-32 stored on hosts 202-3 and 202-4,respectively. From the component placement of targets 502, host 202-1becomes a SN (SN1) for target 502-1, host 202-2 becomes a SN (SN1) fortarget 502-1 and a SN (SN2) for target 502-2, host 202-3 becomes a SN(SN2) for target 502-2 and a SN (SN3) for 502-3, and host 202-4 becomesa SN (SN3) for target 502-3.

VIT module 402 (FIGS. 4, 6, 7 ) on each host 202 can query and monitorthe component placement information from CMMDS 302 (FIG. 3 ) anddetermine if host 202 is a SN for a target. When a host 202 becomes acomponent owner of a target namespace object for a target, the host 202becomes a SN for the target and requests “multi-writer” locks on theLUNs in the target (more specifically the descriptor files of the LUNsin the target namespace object) to read and write them. Unlike theexclusive lock, the VMFS distributed lock mechanism allows multiplehosts 202 to obtain the multi-writer lock on a file so they may read andwrite the same file. When the host 202 loses ownership of the component,the host 202 loses its roles as a SN for the target and closes all iSCSIconnections with initiators and releases the lock on the LUNs in thetarget. If the initiator retries the iSCSI connection, the host 202would redirect the initiator to one of the new SNs for the target.

In the AA architecture, each host is also a DN for all the targets in avSAN cluster. When processing an iSCSI discovery command, a DN in the AAarchitecture returns the IP addresses of all the SNs for a target. Thissimplifies the configuration on the initiator side as the user onlyneeds to configure a single discovery portal since each DN can respondwith all the SNs for any target.

Even if a DN sends the IP addresses of all the SNs for a target to aninitiator, the initiator may sometimes connect to a non-SN because theSNs for the target may occasionally change. In this case, the non-SNredirects the initiator to one of the new SNs for the target. The non-SNrandomly or sequentially selects the redirected SN from the new SNs ofthe target. As previously described, the redirected SN may determineheuristically to further redirect the initiator based on the number ofconnections the redirected SN has for an <initiator, target> pair or itsown workload.

When one SN fails, an initiator still has at least one different path tothe target through another available SN for the target. While it maytake a few seconds for vSAN layer 208 to resume the I/Os in a failover,the time is usually less than 10 seconds. Thus, failover time for iSCSIwill be the same as that for vSAN alone as the iSCSI stack does notintroduce any additional overhead during an AA failover.

FIG. 6 is a block diagram illustrating additional details of system 200in some examples of the present disclosure. Host 202-1 serves as a SNfor a target containing a LUN 602 a. LUN 602 a is backed by a VMDKprovisioned from storage in vSAN datastore 222, such as a local harddisk drive 218 of host machine 202-2 and a local hard disk drive 218 ofhost machine 202-3. Each host 202 includes a VIT module 402 forcommunicating with initiator 406. Hosts 202 uses CMMDS cluster directory604 and configuration file 606 to communicate and store configurationdata for supporting iSCSI targets in vSAN datastore 222.

FIG. 7 is a block diagram illustrating additional details of a host202-1 in some examples of the present disclosure. Host 202-1 includeshypervisor 204-1 having software components that implement vSAN layer208 (FIGS. 4 and 5 ) and the hyper-converged compute layer 206 (FIG. 4). Hypervisor 204-1 includes a kernel 722 that mediates iSCSI I/O accesswith LUNs 602.

Kernel 722 includes VIT module 402-1 to process iSCSI protocols withinhypervisor 204-1. VIT module 402-1 includes an iSCSI protocol processormodule 732 to unpack an inbound iSCSI protocol packet and access a SCSIcommand encapsulated in the inbound iSCSI protocol packet. VIT module402-1 also includes a backend I/O processing module 734 to access a LUN602 identified in the inbound iSCSI protocol packet by using the filesystem commands and protocols of vSAN layer 208. Backend I/O processingmodule 734 may translate between iSCSI protocols and the file systemcommands and protocols of vSAN layer 208 to service iSCSI requests frominitiator 406. As explained above, a LUN 602 consists of storageaggregated from local storage across several hosts 202. Via vSAN layer208, backend I/O processing module 734 accesses those hosts 202 whoselocal storage make up an identified LUN 602.

Host 202-1 has an execution environment called a user space 704 withinwhich various processes of host 202 can execute. For example, user space704 includes a host daemon process (hostd) 744 and a management daemonprocess (vsanmgmtd) 706. Host daemon 744 allows a user (e.g., systemadministrator) to access host 202 directly. Host daemon 744 provides acommand line interface to allow users to configure host 202-1.Management daemon 746 is a server that provides APIs for users toremotely manage host 202-1, such as over the Internet. In accordancewith examples of the present disclosure, host daemon 744 and managementdaemon 746 facilitate the definition and configuration of LUNs 602 to beused as iSCSI targets and to store that information in configurationfile 606.

User space 704 includes a target daemon process (vitd) 742. Targetdaemon 742 handles authentication (e.g., login processing),authorization, and target discovery processing pursuant to the iSCSIprotocol for iSCSI initiators 406. Target daemon 742 may also monitorschanges in and access to information relating to the configuration ofiSCSI targets contained in configuration file 606, and to push thatinformation to kernel 722. Target daemon 742 also interacts with CMMDScluster directory 604 to monitor for changes in the configuration ofiSCSI targets, and to push that information to kernel 722.

Host 202-1 includes a transmission control protocol/Internet protocol(TCP/IP) communication port 706 for communication in accordance withTCP/IP. Communication port 706 may be any suitable network interface(wired or wireless) that can provide a point-to-point connection over aTCP/IP network infrastructure.

Other hosts 202 are configured in the same or similar manner as host202-1.

FIG. 8 is a block diagram illustrating a namespace 800 to manage targetsand LUNs in system 200 (FIGS. 4, 5, and 6 ) in some examples of thepresent disclosure. A namespace object 801 called “iSCSI Targets Home”contains a nested hierarchy of namespace objects for LUNs 602 that areconfigured as iSCSI targets. iSCSI Targets Home 801 includes (1) an“etc” directory 802 to hold configuration file 606 (FIGS. 5 and 6 ) and(2) a “targets” directory 804 to hold symbolic links to target namespaceobjects. Each target namespace object is associated with an iSCSI targetand includes one or more descriptor files that contain information forone or more LUNs 602 (VMDKs) that constitute the iSCSI target. Forexample, (1) a target namespace object 806-1 identifies a “target 1” andincludes descriptor files 808-11 and 808-12 identifying VMDK objects810-11 and 810-12 called “LUN1-UUID.vmdk” and “LUN2-UUID.vmdk” thatconstitute target 1, respectively, (2) a target namespace object 806-2identifies a “target 2” and includes a descriptor file 808-2 identifyinga VMDK object 810-2 called “LUN3-UUID.vmdk” that constitutes target 2,and (3) a target namespace object 806-3 identifies a “target 3” andincludes a descriptor file 808-3 identifying a VMDK object 810-3 called“LUN4-UUID.vmdk” that constitutes target 3. The objects may be relatedby symbolic links to create a hierarchical representation of the objectsthat make up the LUNs and the targets in distributed datastore 222(FIGS. 4, 5, and 6 ).

Every host 202 can query cluster wide information from CMMDS clusterdirectory 604 (implemented with CMMDS agent 302 on each host 202), andevery host 202 can also subscribe to changes for a query. When a host202 updates configuration file 606, the host 202 automaticallyincrements a “config generation number” and publishes it to CMMDScluster directory 604, which notifies the other hosts 202 subscribed toreceive the configuration generation number. When a host 202 receivesthe incremented config generation number, it reads the new configurationfrom configuration file 606, detects any difference between the priorconfiguration in its volatile memory and the new configuration inconfiguration file 606, and makes the appropriate changes. CMMDS clusterdirectory 604 includes a list of subscribed hosts 202 and theconfiguration generation number.

FIG. 9 is a block diagram illustrating a flowchart of a method 900 forsystem 200 to implement AA architecture for VIT service in some examplesof the present disclosure. Method 900, and any method described herein,may be implemented as instructions encoded on a computer-readable mediumthat is to be executed by a processor in a computer system. Method 900,and any method described herein, may include one or more operations,functions, or actions illustrated by one or more blocks. Although theblocks are illustrated in sequential orders, these blocks may also beperformed in parallel, and/or in a different order than those describedherein. In addition, the various blocks may be combined into fewerblocks, divided into additional blocks, and/or eliminated based upon thedesired implementation. Method 900 may begin in block 902.

In block 902, a selected host 202 in vSAN cluster 216 receivesparameters for an iSCSI target 502 and one or more LUNs 602 in thetarget 502. Using vSphere client, vSphere web client, or command lineinterface (CLI), a user accesses vCenter server 212 to set theparameters of target 502 and LUNs 602. For target 502, the user mayprovide a target alias, a virtual network, a TCP port, an authenticationmethod, and a storage policy (e.g., a number of FTT with RAID-1 presetas the failure tolerance method). For each LUN 602, the user may providea LUN ID, a LUN alias, a storage policy (e.g., a number of FTT and afailure tolerance method), and a size. vCenter server 212 randomly,sequentially, or heuristically selects a host 202 from vSAN cluster 216and instructs the selected host 202 to create target 502 and LUNs 602with the parameters.

Instead of using vCenter server 212 to create target 502 and LUNs 602,the user may select a host 202 and use CLI to access the selected host202 (via its host daemon 744) to set the parameters of target 502 andLUNs 602.

Block 902 may be followed by block 904.

In block 904, the selected host 202 creates target 502 and LUNs 602.vSAN module 210 on the selected host 202 creates a target namespaceobject for the target and VMDK objects for LUNs 602 in vSAN datastore222, and publishes their component placement information through CMMDScluster directory 604. For the target namespace object, vSAN module 210creates duplicate components of the object and places them on differenthosts 202 (component owners). VIT module 402 of the selected host 202creates a symbolic link to the target namespace object in the targetsdirectory 804 under the iSCSI Targets Home namespace object 801. Forexample, FIG. 8 shows a symbolic link called “target UUID 3” added totargets directory 804 to add an iSCSI target called “target 3” to thehierarchy of iSCSI targets in the iSCSI Targets Home namespace object801.

For a VMDK object, vSAN module 210 of the selected host 202 createscomponents of the object and place them on different hosts 202(component owners). VIT module 402 of the selected host 202 creates aVMDK descriptor file for the VMDK object in the target namespace object.The VMDK description file includes the UUID and the location of the VMDKobject. For example, FIG. 8 shows a descriptor file 808-3 for the VMDKobject 810-3 called “LUN4-UUID.vmdk” is in the target namespace object806-3 called “target 3.”

Block 904 may be followed by block 906.

In block 906, VIT module 402 of the selected host 202 updatesconfiguration file 606 with information about the newly created target502 and LUNs 602. The information includes identities of the newlycreated target 502 and LUNs 602, and their component owners (queriedfrom CMMDS cluster directory 604). Block 906 may be followed by block908.

In block 908, VIT module 402 of the selected host 202 notifies the otherhosts 202 in vSAN cluster 216 about the newly created target 502, LUNs602 and their component owners. VIT module 402 increments the configgeneration number and publishes it to CMMDS cluster directory 604, whichnotifies the other hosts 202 in vSAN cluster 216. After receiving theincremented config generation number, each of the other hosts 202 (morespecifically its VIT module 402) reads the new configuration fromconfiguration file 606 and learns of the newly created target 502, LUNs602 and their component owners. Block 908 may be followed by block 910.

In block 910, VIT modules 402 of all the hosts 202 determine if they arecomponent owners of the newly created target 502. When a host 202 is acomponent owner of the newly created target 502, that host 202 becomes aSN for target 502 and proceeds to obtain multi-writer locks on LUNs 602(more specifically their descriptor files 808) in target 502 and openthe VMDK objects backing the LUNs 602 in target 502. Any host 202 thatis not a SN for the target does not need to save the identities of theSNs (component owners) as it can query such information from CMMDScluster directory 604 when responding to any iSCSI discovery command fortarget 502.

FIG. 10 is a block diagram illustrating an iSCSI communication sequencein system 200 in some examples of the present disclosure. FIG. 10demonstrates multiple hosts 202 serving as SNs for the same LUN. At step1002, initiator 406 initially performs a discovery operation with hosts202-1 to conduct iSCSI I/Os with a given target 502. At step 1004, host202-1 returns information that identifies hosts 202-2 and 202-3 as SNsfor target 502. Steps 1006 and 1007 demonstrate multipathing throughhosts 202-2 and 202-3. At step 1006, initiator 406 performs a loginsequence with host 202-2 and then performs iSCSI I/Os to a given LUN602. Initiator 406 may send a login request, which contains theidentities of the initiator 406 and the requested target 502,authentication information, if required, and other parameters to benegotiated between the initiator 406 and the target 502. With the loginprocess completed, a session is established for initiator 406 to readand write LUN 602 via host 202-2. At steps 1006 a and 1006 b, host 202-2performs SCSI I/Os with other hosts 202 (e.g., hosts 202-4 and 202-5)whose hard disk drives 218 contain the VMDK backing LUN 602. At step1007, initiator 406 performs a login sequence with host 202-3 and thenperforms iSCSI I/Os to the same LUN 602. With the login processcompleted, a session is established for initiator 406 to read and writeLUN 602 via host 202-3. At steps 1007 a and 1007 b, host 202-3 performsSCSI I/Os with the other hosts 202 (e.g., hosts 202-4 and 202-5) whosehard disk drives 218 make up the VMDK backing LUN 602. Note the sessionwith hosts 202-2 and 202-3 overlap as initiator 406 is accessing LUN 602via both hosts.

vSAN layer 208 uses a quorum mechanism to determine if a host 202 isunreachable to the other hosts 102 in cluster 216. When a host 202 thatalso happens to be a SN for target 502 is unreachable after a specifiedtime (e.g., 60 minutes), vSAN layer 208 creates a new duplicatecomponent of the target namespace object of target 502 and places it ona different host, which becomes a new component owner and therefore anew SN for target 502. For example, when CMMDS 302 of the master nodeuses a quorum mechanism to determine a host 202 (e.g., host 202-2) thatalso happens to be a SN for target 502 is unreachable after a certaintime, the owner DOM 306 of the target namespace object of target 502creates a new duplicate component of the target namespace object andplaces it on a different host (e.g., host 202-1), which becomes a newcomponent owner and therefore a new SN for target 502 previously ownedby host 202-2. DOM 306 publishes the change to CMMDS cluster directory604.

At step 1008, CMMDS 302 of the master node notifies hosts 202 in vSANcluster 216 of a change in the component owners for the target. At step1008 a, each host 202 learn of the new component owner of target 502.For example, host 202-1 may learn it is the new component owner oftarget 502, become a SN for target 502, obtain multi-writer locks onLUNs 602 in target 502, and open the VMDK objects backing the LUNs 602in target 502.

At step 1010, initiator 406 may experience the SN failure by timing outwhile conducting iSCSI I/Os with host 202-2. Note host 202-3 isunaffected by the outage of host 202-2 and continues to serve iSCSI I/Osto target 502. At step 1012, initiator 406 repeats the discoveryoperation with host 202-1 to identify the new SN. At step 1014, host202-1 returns the identity of the new SN (e.g., host 202-1 itself oranother host). At step 1016, initiator 406 performs a login sequencewith host 202-1 and then performs iSCSI I/Os for LUN 602. With the loginprocess completed, a session is established for initiator 406 to readand write LUN 602 via host 202-1 (or another host). At steps 1016 a and1016 b, host 202-1 performs SCSI I/Os with the other hosts 202 (e.g.,hosts 202-4 and 202-5) whose hard disk drives 218 make up the VMDKbacking LUN 602. Note the session with hosts 202-1 and 202-3 overlap asinitiator 406 is accessing LUN 602 via both hosts.

FIG. 11 is a block diagram illustrating an iSCSI communication sequencein system 200 in some examples of the present disclosure. FIG. 11demonstrates a host 202 redirecting initiator 406 to a SN for a targetwhen the host 202 is no longer one of the SNs for the target, and theredirected SN 202 optionally further redirecting the initiator 406 toanother SN for the target. Steps 1002 to 1006 are the same as thosedescribed in FIG. 10 .

In step 1107, initiator 406 sends a login request for target 502 to host202-3. Assume host 202-3 is no longer a SN for the given target 502. Atstep 1108, host 202-3 randomly or sequentially selects one of the SNs oftarget 502 and redirect initiator 406 to the selected SN via iSCSIredirect. For example, host 202-3 randomly selects and redirectsinitiator 406 to hosts 202-1. The redirect is a message informinginitiator 406 that target 502 has moved to a different address so thatinitiator 406 would reissue the login request to the new address.

At step 1110, initiator 406 performs a login sequence with host 202-1and then performs iSCSI I/Os for LUN 602. With the login processcompleted, a session is established for initiator 406 to read and writeLUN 602 via host 202-1 (or another host). At steps 1110 a and 1110 b,host 202-1 performs SCSI I/Os with the other hosts 202 (e.g., hosts202-4 and 202-5) whose hard disk drives 218 make up the VMDK backing LUN602. Note the session with hosts 202-1 and 202-3 overlap as initiator406 is accessing LUN 602 via both hosts.

Instead of step 1110, host 202-1 may further determine heuristically toredirect initiator 406 based on the number of connections host 202-1 hasfor the <initiator, target> pair or its own workload. This isillustrated in optional step 1112, where host 202-1 randomly orsequentially selects one of the SNs of target 502 and redirect initiator406 to the selected SN via iSCSI redirect.

In the AP architecture, there is a single target owner for each targetso SCSI-3 PR requests are processed by one host. To support thefailover, the PR state are persisted to (stored in) a PR file for eachLUN so that the PR state can be reloaded when the target owner failsover.

In contrast to the AP failover architecture, with the AA architecture,all SNs are active for processing I/Os so any of them may receive PRrequests. PR requests consist of SCSI persistence reservation out(PROUT) request that modifies PR state, and SCSI persistence reservationin (PRIN) request that reads the PR state. As PROUT request is rarebecause it is usually only issued by initiators when the membership ofan initiator cluster changes. Thus, the performance impact of processingPROUT request may be ignored. However, I/O fencing requires checking thePR state for each I/O so every SN should fetch the PR state withoutsignificant delay.

In examples of the present disclosure, system 200 leverages CMMDScluster directory 604 and VMFS distributed file lock to synchronize thePR states of LUNs 602 so a new intercommunication channel is not neededfor PR processing. SNs for a target use the VMFS distribution file lockto determine which SN is to lead the processing of a PR request for thetarget, and the SNs use CMMDS cluster directory 605 to communicateparameters for processing the PR request.

FIG. 12 is a block diagram illustrating a method 1200 for system 200 toprocess PR requests in some examples of the present disclosure. FIG. 12demonstrates a host 202 process a PR request and coordinating with theother hosts 202 to update their PR state according. To demonstratemethod 1200, a SN for a target that receives a PR request is hereafterreferred to as “master SN,” and other SNs for the target are hereafterreferred to as “slave SNs.” A master SN is responsible for processingthe PR request and coordinate all SNs for the target. Method 1200 maybegin in step 1.

In step 1, initiator 406 sends a PROUT request for a given LUN 602 to aSN for a target 502 containing the LUN 602. In step 1.1, the SN is thefirst to receive the PROUT request among all the SNs for target 502 andbecomes the master SN immediately. The master SN drains all thein-progress I/Os to LUN 602 and blocks all new I/Os to LUN 602.

In step 1.2, the master SN attempts to acquire the exclusive lock on thePR lock file for LUN 602. A PR lock file 812 (FIG. 8 ) is created foreach LUN 602 in the target namespace object to synchronize PRprocessing. PR lock file 812 provides a mechanism to ensures only one SNprocesses a PROUT request for a LUN within the whole vSAN cluster 216 soonly the SN that gains the exclusive lock on the PR lock file for a LUNmay update the PR state of the LUN in a PR state file 814 (FIG. 8 )shared among hosts 202 in vSAN cluster 216.

Assume in step 1.3, the master SN acquires excusive lock on the PR lockfile for LUN 602. In step 1.4, the master SN publishes a parametercalled “dirty_count,” which indicates the copies of the PR state in theSNs that have not been modified, to CMMDS cluster directory 604 tonotify the slave SNs of the PROUT request. The master SN sets thedirty_count equal to the total number of SNs for target 502. In step1.4.1, CMMDS 302 of the master node pushes the dirty_count to the slaveSNs. In step 1.4.2, the slave SNs start draining all the in-progressI/Os and block any new I/Os. In this phase, the master SN does not needto wait for the slave SNs to finish draining their in-progress I/Osbefore proceeding to step 1.5.

In step 1.5, the master SN updates the PR state of LUN 602 in itsvolatile memory. In step 1.6, the master SN updates the PR state of LUN602 in the PR state file 814. The PR state may include a registered keyand a reservation. In step 1.7, the master SN receives acknowledgementthat the PR state in the PR state file has been successfully updated. Instep 1.8, the master SN decrements the dirty_count in CMMDS clusterdirectory 604. In step 1.9, the master SN resumes I/Os. In step. 1.10,the master SN increments a “PR generation number” and publishes it toCMMDS cluster directory 604 to indicate the PR state has changed. Instep 1.10.1, CMMDS 302 of the master node pushes the incrementedconfiguration generation number to the slave SNs.

In step 1.10.1.1, the slave SNs wait for all the in-progress I/Os to bedrained. In step 1.10.1.2, the slave SNs reload the PR state from the PRstate file to their volatile memories. In step 1.10.1.3, each slave SNdecrements dirty_count in CMMDS cluster directory 604 by one to indicateit has received the latest the PR state. CMMDS ensures that each slaveSN updates dirty_count atomically when multiple SNs try to update theparameter simultaneously. In step 1.10.1.4, the slave SNs resume theirI/O processing.

In step 1.11, the master SN waits for the dirty_count to reach zero (0)in CMMDS cluster directory 604, which indicates all the SNs for target502 have received the latest PR state. In step 1.12, the master SNdetects the dirty_count reached zero (0). In step 1.13, the master SNreleases the exclusive lock on the PR lock file for LUN 602. In step1.14, the master SN receives acknowledgment that the PR lock file hasbeen unlocked. In step 1.15, the master SN responds to the PROUT requestto indicate to initiator 406 that the PR state of LUN 602 has beensuccessfully updated at all the SNs for target 502.

iSCSI supports ordered command delivery from an initiator to a target ina session. VIT service leverages this mechanism for SNs to drain andresume I/Os in method 1100.

If a slave SN fails to reload the latest PR state for a LUN, the slaveSN must fence off all I/Os until it successfully reloads the PR state.Therefore, the master SN defines a timeout for all slave SNs to finishreloading the PR state. When a SN reloads PR state after failover, itwould not decrease the dirty_count. When the master node detects thedirty_count failed to reach zero (0) within the timeout, the master SNassumes one or more slave SNs died and proceeds to steps 1.13, 1.14, and1.15 as usual.

In the case of network partition, the minority node is not able tocontact a component owner so its I/O is fenced off.

As PR requests may be processed by multiple SNs simultaneously, trackingSCSI unit attentions is a challenge as well. Thus, PR state file 814 isextended to persist unit attentions. This enables the slave SNs to readand hand out unit attentions. For example, when a PREEMPT request isreceived, all the paths are notified to read the unit attention otherthan the one where the PREEMPT request is issued.

All the host has the latest SCSI-3 PR state in local volatile memory,they can check whether an I/O is allowed by checking its in-memory PRstate.

From the foregoing, it will be appreciated that various embodiments ofthe present disclosure have been described herein for purposes ofillustration, and that various modifications may be made withoutdeparting from the scope and spirit of the present disclosure.Accordingly, the various embodiments disclosed herein are not intendedto be limiting, with the true scope and spirit being indicated by thefollowing claims.

The invention claimed is:
 1. A method for a hyper-converged, distributedstorage system including a cluster of host computers to implement anactive-active architecture for providing Internet Small Computer SystemInterface (iSCSI) target service, the method comprising: receiving, by afirst host computer in the cluster of host computers, parameters for aniSCSI target and one or more logical unit numbers (LUNs) in the iSCSItarget; creating, by the first host computer, components of the iSCSItarget and placing the components on different component owners selectedfrom the cluster of host computers; providing, by the first hostcomputer, other host computers in the cluster of host computers withinformation about the iSCSI target, the one or more LUNs, and thecomponents owners of the iSCSI target; and concurrently performing, bythe first host computer with at least one of the different componentowners that is active, the iSCSI I/Os with the iSCSI target.
 2. Themethod of claim 1, further comprising, when a storage node for the iSCSItarget has failed and a second host computer in the cluster of hostcomputers is a component owner of the iSCSI target, the second hostcomputer is configured to perform: becoming a new storage node for theiSCSI target; becoming a new discovery node that processes iSCSIdiscovery for the iSCSI target; and while the new storage code isactive, performing the iSCSI I/Os with the iSCSI target concurrentlywith one or more other storage nodes of the iSCSI target.
 3. The methodof claim 1, further comprising: a discovery node in the cluster of hostcomputers receiving an iSCSI discovery command for the iSCSI target; andin response to receiving the iSCSI discovery command, the discovery nodeidentifying all storage nodes of the iSCSI target.
 4. The method ofclaim 1, further comprising: a non-storage node of the iSCSI targetreceiving, from an iSCSI initiator, a login request for the iSCSItarget; and in response to receiving the login request for the iSCSItarget, the non-storage node redirecting the iSCSI initiator to astorage node of the iSCSI target.
 5. The method of claim 1, furthercomprising: a storage node of the iSCSI target receiving, from an iSCSIinitiator, a login request for the iSCSI target; and in response toreceiving the login request for the iSCSI target, the storage node ofthe iSCSI target redirecting the iSCSI initiator to another storage nodeof the iSCSI target.
 6. The method of claim 1, further comprising, whena storage node of the iSCSI target receives a persistent reservation(PR) command to change a PR state of a LUN in the iSCSI target: thestorage node attempting to acquire an exclusive lock on the LUN; inresponse to acquiring the exclusive lock on the LUN, the storage nodeupdating the PR state of the LUN in a PR state file shared by thecluster of host computers; and after the storage node updates the PRstate of the LUN in the PR state file and after all other storage nodesof the iSCSI target reload the PR state of the LUN from the PR statefile, the storage node responding to the PR command.
 7. The method ofclaim 6, further comprising: the storage node draining its in-progressI/Os for the LUN and blocking any new I/Os for the LUN prior toattempting to acquire the exclusive lock on the PR lock file; thestorage node publishing a dirty count parameter to all the storage nodesof the iSCSI target; after updating the PR state of the LUN in the PRstate file, the storage node decrementing the dirty count parameter,republishing the dirty count to all the storage nodes of the iSCSItarget, and publishing a PR generation number to all the storage nodesof the iSCSI target, the PR generation number indicating the PR state ofthe LUN has changed; and after determining the dirty count parameter iszero and prior to responding to the PR command, the storage nodereleasing the exclusive lock on the PR lock file.
 8. A non-transitorycomputer-readable storage medium encoded with instructions providingInternet Small Computer System Interface (iSCSI) target service which,in response to execution by a first host computer in a cluster of hostcomputers, cause the first host computer to: receive parameters for aniSCSI target and one or more logical unit numbers (LUNs) in the iSCSItarget; create components of the iSCSI target and placing the componentson different component owners selected from the cluster of hostcomputers; provide to other host computers in the cluster of hostcomputers with information about the iSCSI target, the one or more LUNs,and the components owners of the iSCSI target; and concurrently performwith at least one of the different component owners that is active, theiSCSI I/Os with the iSCSI target.
 9. The non-transitorycomputer-readable storage medium of claim 8, further comprisinginstructions which, in response to execution by a second host computerin the cluster of host computers when a storage node for the iSCSItarget has failed and the second host computer is a component owner ofthe iSCSI target, cause the second host computer to: become a newstorage node for the iSCSI target; become a new discovery node thatprocesses iSCSI discovery for the iSCSI target; and while the newstorage code is active, perform the iSCSI I/Os with the iSCSI targetconcurrently with one or more other storage nodes of the iSCSI target.10. The non-transitory computer-readable storage medium of claim 8,further comprising instructions which, in response to execution by adiscovery node in the cluster of host computers, cause the discoverynode to: receive an iSCSI discovery command for the iSCSI target; and inresponse to receiving the iSCSI discovery command, identify all storagenodes of the iSCSI target.
 11. The non-transitory computer-readablestorage medium of claim 8, further comprising instructions which, inresponse to execution by a non-storage node of the ISCSI target, causethe non-storage node to: receive from an iSCSI initiator a login requestfor the iSCSI target; and in response to receiving the login request forthe iSCSI target, redirect the iSCSI initiator to a storage node of theiSCSI target.
 12. The non-transitory computer-readable storage medium ofclaim 8, further comprising instructions which, in response to executionby a storage node of the iSCSI target, cause the storage node to:receive from the iSCSI initiator the login request for the iSCSI target;and in response to receiving the login request for the iSCSI target,redirect the iSCSI initiator to another storage node of the iSCSItarget.
 13. The non-transitory computer-readable storage medium of claim8, further comprising instructions which, in response to execution by astorage node of the iSCSI target when the storage node receives apersistent reservation (PR) command to change a PR state of a LUN in theiSCSI target, cause the storage node to: attempt to acquire an exclusivelock on the LUN; in response to acquiring the exclusive lock on the LUN,update the PR state of the LUN in a PR state file shared by the clusterof host computers; and after updating the PR state of the LUN in the PRstate file and after all other storage nodes of the iSCSI target reloadthe PR state of the LUN from the PR state file, respond to the PRcommand.
 14. The non-transitory computer-readable storage medium ofclaim 13, further comprising instructions which, in response toexecution by the storage node, cause the storage node to: drain itsin-progress I/Os for the LUN and blocking any new I/Os for the LUN priorto attempting to acquire the exclusive lock on the PR lock file; publisha dirty count parameter to all the storage nodes of the iSCSI target;after updating the PR state of the LUN in the PR state file: decrementthe dirty count parameter; republishing the dirty count to all thestorage nodes of the iSCSI target; and publish a PR generation number toall the storage nodes of the iSCSI target, the PR generation numberindicating the PR state of the LUN has changed; and after determiningthe dirty count parameter is zero and prior to responding to the PRcommand, release the exclusive lock on the PR lock file.
 15. Ahyper-converged, distributed storage system configured to support anactive-active architecture for providing Internet Small Computer SystemInterface (iSCSI) target service, comprising: a cluster of hostcomputers, each including a hypervisor, wherein a first host computer inthe cluster of host computers is configured to: receive parameters foran iSCSI target and one or more logical unit numbers (LUNs) in the iSCSItarget; create components of the iSCSI target and place the componentson different component owners selected from the cluster of hostcomputers, wherein each of the different component owners corresponds toa storage node for the iSCSI target; create each LUN from storagedistributed across the cluster of host computers; provide other hostcomputers in the cluster of host computers with information about theiSCSI target, the one or more LUNs, and the components owners of theiSCSI target; and concurrently perform, with at least one of thedifferent component owners that is active, the iSCSI I/Os with the iSCSItarget.
 16. The system of claim 15, wherein, when a storage node for theiSCSI target has failed and a second host computer in the cluster ofhost computers is a component owner of the iSCSI target, the second hostcomputer is configured to: become a new storage node for the iSCSItarget; become a new discovery node that processes iSCSI discovery forthe iSCSI target; and while the new storage code is active, perform theiSCSI I/Os with the iSCSI target concurrently with one or more otherstorage nodes of the iSCSI target.
 17. The system of claim 15, wherein:each host computer in the cluster of host computers is configured tobecome a discover node that receives an iSCSI discovery command for theiSCSI target; and in response to receiving the iSCSI discovery command,the discovery node is configured to identify all storage nodes of theiSCSI target.
 18. The system of claim 15, wherein: a non-storage node ofthe iSCSI target is configured to receive, from an iSCSI initiator, alogin request for the iSCSI target; and in response to receiving thelogin request for the iSCSI target, the non-storage node is configuredto redirect the iSCSI initiator to a storage node of the iSCSI target.19. The system of claim 15, wherein: a storage node of the iSCSI targetis configured to receive, from the iSCSI initiator, the login requestfor the iSCSI target; and in response to receiving the login request forthe iSCSI target, the storage node of the iSCSI target is configured toredirect the iSCSI initiator to another storage node of the iSCSItarget.
 20. The system of claim 15, wherein, when a storage node of theiSCSI target receives a persistent reservation (PR) command to change aPR state of a LUN in the iSCSI target: the storage node is configured toattempt to acquire an exclusive lock on the LUN; in response toacquiring the exclusive lock on the LUN, the storage node is configuredto update the PR state of the LUN in a PR state file shared by thecluster of host computers; and after the storage node updates the PRstate of the LUN in the PR state file and after all other storage nodesof the iSCSI target reload the PR state of the LUN from the PR statefile, the storage node is configured to respond to the PR command. 21.The system of claim 20, wherein the storage note is configured to: drainits in-progress I/Os for the LUN and blocking any new I/Os for the LUNprior to attempting to acquire the exclusive lock on the PR lock file;publish a dirty count parameter to all the storage nodes of the iSCSItarget; after updating the PR state of the LUN in the PR state file,decrement the dirty count parameter, republish the dirty count to allthe storage nodes of the iSCSI target, and publish a PR generationnumber to all the storage nodes of the iSCSI target, the PR generationnumber indicating the PR state of the LUN has changed; and afterdetermining the dirty count parameter is zero and prior to responding tothe PR command, release the exclusive lock on the PR lock file.